The most striking difference between radio drama and the ordinary stage play is that in the former everything the characters say is heard primarily as a soliloquy.  In a stage play the audience ‘overhear’, so to speak, the remarks which the actors they see address to each other; in a radio play each remark is heard as addressed directly to the listener, and its effect upon the invisible characters in the play is secondary.


W H Auden, Foreword to Persons from Porlock and Other Plays for Radio, BBC (1969)